Journal of Physical Chemistry A, Vol.113, No.31, 8852-8856, 2009
Anomalously Selective Quenching of S-2 Fluorescence from Upper Excited State of Zinc 5-(1'-Pyrenyl)-10,15,20-triphenylporphyrin Derivatives through Intramolecular Charge Transfer State
Zinc 5-(1'-pyrenyl)-10,15,20-tris(p-methoxyphenyl)porphyrin (ZnP-Py) and zinc 5-(1'-pyrenyl)-10,15,20-tris(pentafluorophenyl)porphyrin (ZnFP-Py), in which a pyrenyl group is directly connected at the meso-positions of their corresponding porphyrins, were synthesized and investigated on the photophysical properties. While the S-1 fluorescence of the porphyrin moieties was not quenched by the pyrenyl group at all, the S-2 fluorescence was remarkably quenched. The anomalously selective quenching of the S-2 fluorescence was dependent on the solvent polarity, and the quenching was enhanced in the polar solvent. The S-2 fluorescence quenching is considered to take place through an intramolecular charge transfer state. Interestingly, the charge transfer state relaxes to the S-1 state through the rapid charge recombination with high quantum yield. The charge transfer rate of ZnFP-Py, which was calculated from the S-2 fluorescence quenching, was larger than that of ZnP-Py, possibly due to the differences in the free-energy changes of the electron transfer and the electron densities at their meso-positions. This study has suggested that ultrafast electron transfer from the upper excited state and charge recombination to S-1 can occur in a directly connected electron donor-acceptor system.